There is a technology to apply an outline micro character to a character portion printed with a black (K) ink, and a technology to print the micro character with a mixed color of cyan (C), magenta (M) and yellow (Y) inks. Since an outline micro character is small in a character size, it has characteristics that to print it by a general printer is difficult, and to visually recognize it is also difficult. In addition, a micro character printed with a mixed color of CMY inks has characteristics that to visually recognize it is further difficult.
However, a technology to apply a micro character is a technology which is limited to a character portion printed with a K ink, and it may be sometimes difficult to embed forgery preventing information, such as a micro character, in an image in which black portions thereof are indefinite, such as a face image.